The present invention relates generally to the field of integrated circuit (IC) fabrication and, more particularly, to a ferroelastic lead germanate film and metal organic chemical vapor deposition (MOCVD) method for the above-mentioned thin film.
Ferroelastic films have attracted great interest in recent years because of their potential for use in applications such as high-energy storage capacitors and high-strain actuators/transducers. More recently, with the development of microelectronic devices, ferroelastic thin films have been explored for use in microactuators, microelectromechanical systems (MEMS), and as decoupling capacitors in high speed multichip modules (MCMs). It has been found that for charge storage applications, ferroelastics are superior to ferroelectrics because the stored charge can be completely released due to absence of remanent polarization. Ferroelastic films are superior to films with linear dielectrics because of their high dielectric constant and high charge storage density. However, it is difficult to obtain ferroelastic thin films with square hysteresis loops and zero remanent polarization.
The fabrication and characterization of ferroelastic lead germanium oxide thin films (PGO), especially Pb3GeO5 are of current interest. Lead germanate is a relative new materials. The PbOxe2x80x94GeO2 binary system has been studied by Speranskaya (1959). R. R. Neurgaonkar et al. grew single crystal of ferroelastic Pb3GeO5 by the Czochralski technique (1974). The ferroelastic properties in this material were first discovered by R. R. Neurgaonkar et al. The dielectric and electric-optics properties of single crystal and polycrystalline materials have been reported in the literature. The ferroelastic Pb3GeO5 belongs to the monoclinic space group P2 at room temperature. The crystals are ferroelastic, but show no phase transitions up to the melting point (738xc2x0 C.). The interesting feature of this material is that Pb3GeO5 has ferroelastic properties, which are suitable for microelectromechanical system (MEMS) applications.
Ferroelectric lead germanate (Pb5Ge3O11) thin films have been made by thermal evaporation and flash evaporation (A. Mansingh et al., 1980), dc reactive sputtering (H. Schmitt et al., 1984), laser ablation (S. B. Krupanidhi et al., 1991 and C. J. Peng et al., 1992), and sol-gel technique (J. J. Lee et al., 1992). However, ferroelastic Pb3GeO5 thin films made by MOCVD processes have not been reported.
The present invention PGO film has ferroelastic properties that are useful in microelectromechanical system (MEMS) and high speed multichip module (MCM) applications. In co-pending patent application Ser. No. 09/301,435, entitled xe2x80x9cMulti-Phase Lead Germanate Film and Deposition Methodxe2x80x9d, invented by Tingkai Li et al., filed on Apr. 28, 1999, attorney docket No. SLA400, a second phase of Pb3GeO5 is added to the Pb5Ge3O11, increasing grain sizes without an increase in c-axis orientation. The resultant film has increased Pr values and dielectric constants, and decreased Ec values. Such a film is useful in MEM, MCM, DRAM, and FeRAM applications.
In co-pending patent application Ser. No. 09/301,420, entitled xe2x80x9cC-Axis Oriented Lead Germanate Film and Deposition Method for Samexe2x80x9d, invented by Tingkai Li et al., filed on Apr. 28, 1999, attorney docket No. SLA401, the Pb5Ge3O11, film is crystallographically oriented in the c-axis. This film has smaller Pr and dielectric constant values, and is useful in one transistor (1T) applications.
In co-pending patent application Ser. No. 09/302,272, entitled xe2x80x9cEpitaxially Grown Lead Germanate Film and Deposition Methodxe2x80x9d, invented by Tingkai Li et al., filed on Apr. 28, 1999, attorney docket No. SLA402, now U.S. Pat. No. 6,190,925, an epitaxial grown PGO film is disclosed with extremely high c-axis orientation. As a result, high Pr and Ec values, as well as lower dielectric constant, is obtained. Such a film is useful in 1T, and one transistor/one capacitor (1T/1C) FeRAM applications. The three above-mentioned co-pending patent applications are incorporated herein by reference.
It would be advantageous if a CVD process could be developed for the deposition of ferroelastic PGO thin films.
It would be advantageous if a CVD process, offering the advantages of excellent film uniformity, compositional control, high film densities, high deposition rates, excellent step coverage, and commercial amenability, could be developed for PGO processes.
Accordingly, a method for forming a ferroelastic lead germanium oxide (PGO) film on an integrated circuit (IC) IC wafer has been provided. Typically, the wafer is at least partially covered with a conductive electrode material of Ir or Pt. The method comprises the steps of:
a) mixing [Pb(thd)2] and [Ge(ETO)4] to form a PGO mixture having a molar ratio of about 3:1;
b) dissolving the mixture of Step a) with a solvent of tetrahydrofuran, isopropanol, and tetraglyme, having a molar ratio of about 8:2:1, respectively, to form a precursor solution having a concentration of approximately 0.1 to 0.5 moles of PGO mixture per liter of solvent;
c) introducing the precursor solution to a precursor vaporizer at a rate in the range of approximately 0.1 to 0.5 milliliters per minute (ml/min), and heating the solution to create a precursor gas having a temperature in the range of approximately 180 to 250 degrees C. and a precursor vapor pressure in the range of approximately 30 to 50 torr (T);
c1) mixing the precursor gas in the chamber with an argon gas shroud flow in the range of approximately 4000 to 6000 square cubic centimeters per minute (sccm), preheated to a temperature in the range of approximately 170 to 250 degrees C.;
c2) introducing an oxygen flow to the reactor in the range of approximately 1000 to 3000 sccm, whereby a lead-germanium oxide with a c-axis orientation is promoted;
d) heating the wafer chuck to a temperature in the range of approximately 500 to 650 degrees C., establishing a reactor chamber pressure in the range of approximately 5 to 10 T, and decomposing the precursor gas on the IC wafer to form a PGO thin film with a thickness of less than 1 millimeter (mm), including a first phase of Pb3GeO5, whereby the PGO film having ferroelastic properties is formed;
e) cooling the PGO film to approximately room temperature in an oxygen atmosphere; and
f) annealing the PGO film formed in Step d) in an atmosphere selected from the group of oxygen and oxygen with Pb atmospheres, with the oxygen being introduced at a partial pressure greater than approximately 20%, whereby the ferroelastic properties of the PGO film are improved.
In some aspects of the invention further steps follow Step f) of:
g) forming a conductive electrode overlying the PGO film; and
h) annealing the PGO film in an atmosphere selected from the group of oxygen and oxygen with Pb atmospheres, with the oxygen being introduced at a partial pressure greater than approximately 20%, whereby the interface between the PGO film and the electrode formed in Step g), is improved.
Typically, Steps f) and h) include using a rapid thermal annealing (RTA) process to anneal the PGO film, in which the temperatures are in the range of approximately 500 to 750 degrees C., for a duration in the range of approximately 10 to 30 minutes, and a thermal temperature ramp-up in the range of approximately 10 to 200 degrees C. per second. Alternately, furnace annealing is performed at temperatures between 500 and 600 degrees for time durations of 30 minutes to 2 hours.
A lead germanium oxide (PGO) thin film having ferroelastic properties is also provided. The PGO film comprises a first phase of Pb3GeO5, with the thickness of the first phase of Pb3GeO5 being less than approximately 1 mm, whereby said Pb3GeO5 phase improves the ferroelastic properties of the PGO film. Typically, the first phase Pb3GeO5 has a grain size in the range of approximately 1 to 2 microns.
Also provided is a capacitor. The capacitor comprises a first conductive electrode, a PGO thin film including a first phase of Pb3GeO5 overlying the first electrode, and a second conductive electrode overlying the PGO film, whereby a PGO film capacitor is formed having ferroelastic properties. The capacitor has a dielectric constant in the range of approximately 50 to 100, and a leakage current of 4xc3x9710xe2x88x926 A/cm2 at 100 kV/cm. The minimum polarization voltage is approximately 1 volt, and the saturation voltage is less than approximately 5 volts.